This invention relates to nuclear magnetic resonance (NMR) imaging systems and methods and, more particularly, to systems and methods which compensate NMR image quality for temperature effects on the NMR system magnet.
In the past, the NMR phenomenon has been utilized by structural chemists to study, in vitro, the molecular structure of organic molecules. Typically, NMR spectrometers utilized for this purpose were designed to accommodate relatively small samples of the substance to be studied. More recently, however, NMR has been developed into an imaging modality utilized to obtain images of anatomical features of live human subjects, for example. Such images depicting parameters associated with nuclear spins (typically spins of hydrogen protons associated with water in tissue) may be of medical diagnostic value in determining the state of health of tissue in the region examined. NMR techniques have also been extended to in vivo spectroscopy of such elements as phosphorus and carbon, for example, providing researchers with tools, for the first time, to study chemical processes in a living organism. The use of NMR to produce images and spectroscopic studies of the human body has necessitated use of specifically designed system components, such as the magnet, gradient and RF coils.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons or neutrons. Due to the spin of the protons and neutrons, each such nucleus exhibits a magnetic moment such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field B.sub.0, a majority of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.0, the aligned magnetic moments precess about the axis of the field at a frequency dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency .omega., also referred to as the Larmor frequency, is given by the Larmor equation .omega.=.gamma.B in which .gamma. is the gyromagnetic ratio (which is constant for each NMR isotope) and wherein B is the magnetic field (B.sub.0 plus other fields) acting upon the nuclear spins. It is thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.0, may be perturbed by the application of magnetic fields oscillating at or near the Larmor frequency. Typically, such magnetic fields designated B.sub.1, are generated orthogonally to the direction of the B.sub.0 field by RF pulses supplied through a coil connected to an RF transmitting apparatus. Under the influence of RF excitation, magnetization M rotates about the direction of the B.sub.1, field. In NMR studies, it is typically desired to apply RF pulses of sufficient magnitude and duration to rotate magnetization M into a plane perpendicular to the direction of the B.sub.0 field. This plane is commonly referred to as the transverse plane. Upon cessation of the RF excitation, the nuclear moments rotated into the transverse plane precess around the direction of the static field. The vector sum of the spins forms a precessing bulk magnetization which can be sensed by an RF coil. The signals sensed by the RF coil, termed NMR signals, are characteristic of the magnetic field and of the particular chemical environment in which the nuclei are situated. In magnetic resonance imaging (MRI) systems, which are systems that employ NMR imaging, the NMR signals are observed in the presence of magnetic-field gradients which are utilized to encode spatial information into the signals. This information is later used to reconstruct images of the object studied in a manner well-known to those skilled in the art.
A common NMR imaging problem results from the temperature dependent nature concerning operation of NMR magnetic sources, such as a permanent magnet used to produce the B.sub.0 field (the "B.sub.0 magnet"). That is, temperature changes in the B.sub.0 magnet alter the strength of the otherwise static B.sub.0 field. Temperature changes are the ordinary consequence of temperature gradients in a testing room, such as may result from localized positioning of warm lights or air conditioning/heating vents. Temperature gradients may cause different parts of the magnet to have different temperatures. For example, warm lighting located in the ceiling may cause an upper part of a magnet to be warmer than its corresponding lower portion. Alternatively, the entire magnet may have the same temperature, but one that changes over time, such as when a room heats up or cools down over the course of a day.
Regardless of whether the B.sub.0 magnet is subjected to localized or generalized temperature variation, it is desirable for NMR imaging to produce a homogeneous B.sub.0 field of precise strength, typically for extended periods of time. However, normal temperature changes in the B.sub.0 magnet (as discussed above) lead to undesirable variations in B.sub.0 field strength, which changes the Larmor frequency, resulting in image degradation.
What is needed is a system and method to compensate for temperature changes in the B.sub.0 magnet, thereby improving NMR image quality.